From Kotaku:
Guess that's why it was listed with WFC.
The player who edits their level is ultimately creating an obstacle course for whoever plays that level once it is shared from DS to DS.
The goal, as in the pre-made stages of the game, will be to collect a star. But what will stand in their way, and how the shared world will behave, will be a surprise, courtesy of someone else.
Players start with pre-made terrain. They can write whatever they want into the game — as is standard with Scribblenauts — and it will appear. Writing a second thing into the level produces the opportunity to define the relationship between the two things that the level-creating player has put on the screen. So if you've written a bear and some honey into the game world, the bear can be made to fear honey instead of craving it. The producer demonstrating the game to me showed me a simple word-balloon-style menu that has these relationships listed in it. Pick the one you want and that will be the law in that level.
Guess that's why it was listed with WFC.